[Managarm]

From my little newbie experience with both MA Marignon and Abysia, I'd recommend starting with Marignon, because it plays like a more conventional nation, having plenty of options and answers to (almost) everything

In terms of units, you can fulfill all the battlefield main roles: missile support (Crossbowman), expendable front liners (Flagellants), heavy infantry (Man At Arms) and very good (although expensive) cavalry (Royal Guard/Knights Of The Chalice). On top of that, Marignon also have plenty of priestly/magic powers to unleash and some tactical warheads like Assassins and Spies; although Abysia has got the superior assassins there.
Marignon can also benefit a lot from a good fire bless and dominion: Flagellants are dirty cheap, can be recruited everywhere and combining two attacks plus Flaming Weapons it's pure evil! Knights Of The Chalice also become very nasty with the Flaming Weapons feature, BTW.

In terms of magic, Grand Masters have Fire 3/Astral 2 base plus two additional random paths, and one it's 100%. Recruiting some of them (unfortunately they're capital only) you reach F4 or S3 easily without any empowerment or magic item.
F3 grants you fully access to Evocation gems such as Falling Fires, Flare, Fireball, Fire Cloud and Holy Pyre (National Spell)... You can also cast Flaming Arrows (Enchantment, F3 required) giving an edge to your Crossbowmen/indy archers. Enchantment also gives you the chance to make your troops flame immune thanks to Fire Fend/Warriors of Muspelheim.
Another worth mentioning spells, from Thaumaturgy school, are Raging Hearts (F4) for crippling enemies' economy alongside with Spies, and Soul Slay (S3) a basic anti-SC spell. If you go Astral you can also try some of the outstanding National Summons...

High Inquisitors are level 3 priests capable of Divine Blessing/Smite all day, and they're also very effective at killing enemy dominions

The biggest drawback they've is the ageing problems that the best units suffer (Grand Masters/High Inquisitors)

That's in a nutshell what can Marignon do for you...